For the pair of linear equations a1x + b1y + c1=0 and a2x + b2y + c2=0, the solution set are x=(b1c2−b2c1)(a1b2−a2b1) and y=(c1a2−c2a1)(a1b2−a2b1). They can also be written as:
x/b1c2−b2c1 = y/a2c1−a1c2 = 1/b2a1−b1a2
We have x=b1c2−b2c1(a1b2−a2b1 and y=c1a2−c2a1a1b2−a2b1
We can write this as xb1c2−b2c1 = yc1a2−c2a1 = 1a1b2−a2b1
This is the basis for the cross multiplication method.